1. Field of the Invention
The present invention relates to an image forming process, particularly, to a dry process for recording images using a photopolymerizable material.
2. Description of the Prior Art
Hithereto, as photoresist materials used for producing printed circuits, recording materials which are subjected to solution development, using, for example, a photosensitive solution comprising a bichromate and glue or polyvinyl alcohol, a photosensitive solution comprising polyvinyl cinnamate and a photosensitizing agent, or a photosensitive solution comprising natural rubber or cyclized rubber and a cross linking agent as main ingredients have been widely used. Further, there are known photoresist materials which comprise a photosensitive material inserted between two plastic films. In such photoresist materials, one of the plastic films is stripped off to expose the photosensitive material upon use. After being superposed on a desired substrate for a printed circuit, the photosensitive layer is image-wise exposed to light through the remaining plastic film, whereafter the plastic film is removed and the photosensitive layer is developed by a suitable developer such as an organic solvent or an aqueous alkali solution, etc. The photosensitive layer hardened by exposure to light remains on the printed circuit substrate while non-exposed portions are removed by dissolution in a developer to form a resist pattern for a printed circuit. However, processing solution development type recording materials is complicated because a developing solution must be used. Further, waste developer liquids pose a serious danger of environmental pollution.
Recently, dry development image recording material and processes have been suggested, e.g., as described in Japanese Patent Publications 9663/63 and 22901/68, Japanese Patent Publication (OPI) 7728/72, Japanese Patent Publication 43126/73 and Japanese Patent Publication (OPI) 33623/72, there are processes for forming images utilizing the property that the order of the adhesive strength of a resist layer in exposed areas to the support and to the printed circuit substrate is the reverse of that of the resist layer in non-exposed areas. This process generally comprises applying a photopolymerizable composition (photosensitive layer) consisting of an addition polymerizable monomer, a light polymerization initiator and a binder to a support, such as a thin transparent plastic film, contacting the photosensitive layer with a transparent or opaque substrate such as a metal plate, a plastic plate or film, or paper, etc., image-wise exposing to light through the transparent support using an original and stripping off the support to leave either the exposed portions or the non-exposed portions of the photosensitive layer on the PC substrate or the support, whereby negative images and positive images are formed on the substrate and the support. According to this recording process, images such as a resist pattern can be formed merely by stripping off the support because liquid development is unnecessary. Accordingly, the production process per se of printed circuits is rendered more economical. Further, there is no danger of environmental pollution, as waste liquids are not discharged.
In the production of resist patterns on printed circuit substrates, in some cases the whole portion of the resist pattern is closely adhered to a substrate, such as a copper plate, superposed on a plastic plate. On the other hand, in the case of producing printed circuits using a substrate having through-holes wherein both surfaces of the substrate and the inside faces of the through-holes are covered with a metal layer by which the circuits formed on both surfaces of the substrate are in electrical communication via the inside faces of the through-holes, it is necessary to form a resist film over the through-holes without contact between the resist film and the entire metal surface ( called tenting ). Tenting is necessary to prevent etching of the metal layer on the inside faces of the through-holes when printed circuits are formed by etching. In the case of tenting, it is required that the formed resist film adhere closely to the substrate at only very small areas around the through-holes while other portions of the resist film be supported by the cohesive force of the film over the through-holes and such a film have a strength sufficient to withstand the mechanical shock due to the spraying pressure of a spray type etching process. Accordingly, the properties of materials for forming a resist film, particularly properties such as the film strength of the polymer film used as a binder, are important.
In stripping development recording materials suggested hitherto, polymers such as chlorinated polyolefins (see Japanese Patent Application 97049/73) or chlorinated rubber (see Japanese Patent Publication (OPI) 7728/72) have been exclusively used as the binder for obtaining images of excellent quality. Although the resist film using such polymers has a sufficient antietching capability in the case that the total film is closely adhered to the base plate, it does not have a film strength sufficient to withstand a spraying pressure at etching in the case of use for tenting, and, consequently, the tented resist film laid over the through-holes of the substrate is broken during etching.
Properties required of a resist film are described in Japanese Patent Application 97049/73, that is, the adhesive strength between the resist layer which remains on the substrate and the substrate must be sufficiently high at stripping development so that the resist layer is not stripped off due to the resist layer to be removed. On the other hand, it is further necessary that the adhesive strength between the resist layer which remains and the support to be removed be low. Further, it is preferred that the resist layer be sharply broken at boundary lines between exposed areas and non-exposed areas, whereby sharp resist images are formed on the substrate. According to the result of studies, we have found that a resist layer which forms such sharp images often has the fault that tenting film strength is low, and the resist film on the through-hole portions is broken during etching.